Unusual Warp Switches Looming in Textile Factory

Some time ago I was called to a textile factory to investigate some fairly new water-jet weaving looms it had purchased secondhand, which had an intermittent fault that was driving the in-house technicians to distraction.

Each loom had hundreds of electronic sensor-switches that detected warp threads breaking while weaving cloth. When a thread snapped, the loom stopped automatically and awaited a weaver to tie the thread and restart the machine. This prevented the machine from just carrying on and making huge amounts of scrap cloth.

The problem annoying the maintenance technicians was that in four cases out of 10, when a thread snapped, the weaving loom wouldn’t restart after the thread was retied. The machine had to be completely powered down to reboot the control box, which took a good 10 minutes as various capacitors had to be discharged. Threads snapped, on average, five times a day on each machine, and there were 54 identical looms. Ten minutes is an awfully long time in a production schedule. Multiply this by the number of breakages times, the number of looms, and the fail-to-restart factor, and you get an idea of how much downtime the factory was experiencing. The local management was seriously considering scrapping the machines because of the losses they were incurring.

My first problem was that the machinery came without maintenance documentation. The local team had not expected serious technical differences to their existing equipment as it looked superficially similar, and they accepted the machines after they saw them running.

As the obvious causes were the sensors-switches and the control box, these were the first places to look. I hooked up a logic analyzer to a machine, started it up, and cut a thread to simulate a break. The weaver retied the thread and started the machine. We had to do this a few times to generate a fault.

The logic analyzer showed the fault was in the switch, which sent a signal to the control box and then stuck in the open mode. I reset the machine and 10 minutes later tried again. This time the switch stuck in the closed mode.

A few tests demonstrated that the switch always sent out a pulse when it sensed a break but needed an external signal to reset properly. A little delving into the control box soon showed that this had been forgotten in the design -- there was no reset signal generated at all. My guess is this was the reason the looms were sold on the secondhand market in such good condition. An oddity was that the switch needed a trailing then a leading edge 20ms or more apart to reset.

Working overnight with the technicians, I installed a circuit that sent a logic low of 50ms to all the sensor-switches and delayed the restart signal by 100ms when the restart button was pressed on all the machines. At the same time I installed a capacitor discharge circuit into every control box, consisting of a 10k 2W resistor, an LED, and a momentary contact switch. This reduced the maintenance reset time to less than 30 seconds.

About a month later I called the factory to ask how things were going, and was told that the problem had gone away.

This entry was submitted by Ian Proffitt and edited by Lauren Muskett.

That is a risky addition to the system. I had to add a similar circuit to a CNC motion controller's e-stop. I needed the controller's onboard capacitors to drain instantly. I wanted the motors to stop dead.

A few years ago when PCs were fairly new I did the electronics for a machine that utilized that I/O system having boards tied to a 64 conductor ribbon cable as the interface I/O bus. There was a huge design flaw in that if the control computer crashed, or stopped addressing the bus, all of the outputs would stay as they had last been set. Since this was a wet ful stand, that meant that gasoline would continue to flow if the system locked up, which happened daily while the testing program was being developed. The solution was a simple timer to pull down the master reset line whan it timed out after 1 second. The timer would be reset every 100milliseconds by the I/O write pulse if the control program was running, so under normal conditions the timer never timed out. We put the timer on the digital output board, since that was where control was most critical. But I never did find out why the makers of that system didn't even warn about the potential hazard.

Isn't it amazing how expensive a simple design error like this can be to the equipment owner? The guys that design the system probably never realized that a broken thread was a normal event that happened frequently.

That is like forgetting to connect the brake pedal on a new car- it steers fine, it accelerates fine, and the radio plays all your favorite stations. There's just this one little problem, so we will sell it to you cheap!

This post points to two of my biggest complaints. not only in industry, but everyday life as well. First is the lack of supporting documentation. I have griped about this before on this forum, so I will again. When I/we have spent thousands to purchase a piece of equipment or a software package they should come with an operators manual, including a section on trouble shooting. I would even be willing to pay an addtional amount, but instead you are lucky to get a web site or a phone number which is often peopled by someone who speaks with such an accent that everything after the initial greeting is impossible to understand.

The second and worst is a dealer who will resell defective equipment. It is impossible to believe these problems had not occurred before. The buyer should have been warned. If the original owner hid the problem from the reseller, than shame on him as well. What has happened to pride and integrity?

The quickest way to get deleted from my company's list of vendors is to supply defective parts or equipment and then not make the situation right. Particularly if the defect was known in advance. I wish the author of this article would have posted the name of the vendor to expose them for the frauds they appear to be.

Buying "defective" equipment isn't always a bad deal. A friend go a really good price for a radial arm saw. Apparently it would stall and trip on overload whenever the owner tried to cut something. It took only a few minutes to check the connection box and determine that the jumpers were set to 240 VAC but the cord was plugged into 120 VAC.

Chances are your friend knew in advance there was a defect, hence the reduced price. This writer does not say he purposely bought some thing defective so he could save a few bucks.

Many years ago I was looking over a clearance table in a sporting goods store and found a fishing reel at half price, marked `AS IS'. I cleared a spot on the table and dismantled the reel and did not find anything wrong so I bought it. It has served me faithfully for 20+ years. However, at the checkout another man approached me and asked if I knew what was wrong with the reel. I told him I could not find a problem and he told me, "I took it apart and couldn't figure out what was wrong, so I was afraid to buy it."

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